Controlled atmosphere for heattreating metals



21, 1951 I H. H. CHlSWlK I 4 2,565,201

CONTROLLED ATMOSPHERE FOR HEAT-TREATING METALS Filed Sept. 26, 1947 SUPPLY OF N2 AND 02 CHARCOAL REACTOR ANNEALING FURNAQE INVENTOR HA| M H. CHISWIK BY 4 V... dawn, 2M4; 604m ATTORNEYS Patented Aug. 21, 1951 UNIT ED -s -TA-T-Es {oer-r ce commutes ATMos'i? iii-antes firm- TREATING METALS Harm H. 'Chiswik," East orange, N. -"J.-';" "assignor' to Air- Reduction Company, Incorporated, N w York, N. Y., a corporation"ofNcwYork Application September 261947, Serial N6.7"76',"275

metal treated. Although principally concerned 'withthe annealing of steelfit is applicable to the treatment of other metals which are subject to oxidation or other modification at high temperatures. 1

-. It has been suggestedheretofo're that annealing may be accomplished in the presence ofan atmosphere of nitrogen containing carbon 'dioxide and carbon monoxide. In accordance with the earlier procedura'nitrogen containing not morethan' about 5% of oxygen is passedthrough carbon which must 'be heated to a temperature substantially in excess of thetemperature em played in theseannealing furnace. Whatever merit the procedure-may have, it obviously in- When steel, for example, of given carbon content, is heat treated at a given temperature in the presence of gas containing nitrogen, carbon monoxide and" carbon dioxide, the carburizing or I decarburizing effect isgoverned by the relative concentrations of carbon monoxide and carbon dioxide in the gas. This is expressed'by the ratio (Peer P00,

in which Poo and Poo, arethepartial pressures which are proportional to concentration. If the ratio is greater than some fixed value expressed as K (the equilibrium constant), the steel will be carburized. If the ratio is less than K, the

steel will be 'decarbur-ized. If the ratio is equal to K, the result is neither carburization nor docarburization. The value of K differs, depending upon the carbon content of the steel and the heat treating temperature. Thus, for 0.40% carbon steel, heat treated at 800 C., K =3. At 900 2- Claims. (01. 148-16.7)

C'., K =11. For'an1 0.'8 0% carbon'steel, the corresponding values of Kare 6 and 20', r'espeetwely.

Similarly, when a gas consisting of nitrogen and oxygen is passed over charcoal atfaf given temperature and equilibrium 'is'established, the resulting gas will consist of nitrogen, carbon monoxide and carbon dioxide in which the relative concentrations of carbon monoxide and carbon dioxide are fixed, so'thatthe ratio is constant. Thus, a gascontainingnitrogen and oxygen, when passedjoyer. charcoal at '700 (2., will produce a gas containing 'carbon'monoxide and carbon dioxide in proportions such that ratio 00) ft oo If the charcoal is at 1000 0., the ratiois 146.5. The result isthe same, regardless of "the percentage of oxygen'in the original gas.

Advantage is taken of these facts in accordance with the present invention by' producing the desired gaseous atmosphere-at any given temperature, regardless of'the' temperaturefof annealing, and then modifying the composition of the gas to the ratio desired'for annealing of the particular metal at the predetermined temperature. The carbon bed may thus be operated at a temperature varying over al'wide rangefbut preferably between 500 and 1200 C. Adjustment of the ratio of carbon monoxide and carbon dioxide is then effected readily by selectively absorbing either carbon monoxide or carbon dioxide until the desired ratio of carbon monoxide to carbon dioxide forthe annealing atmosphere is attained. Thus, if the gas produced haslan excess of carbon monoxide, it is passed through a carbon monoxide absorber. If the carbon dioxide is too high, the gas is passed through a carbon dioxide absorber. Only a portion of "the gas need be treated ordinarily, and the'balance is bypassed to the annealing furnace, being mixed with the modified gas to produce an atmosphere of the desired composition.

As a result of this procedure, it is possible to avoid the difiiculties hereinbeforementioned, and

particularly that of maintaining the carbonbr charcoal at a predetermined temperature inexcess of that employed intheannea'li'ng"furnace. It is much easier to attain and maintain an atmosphere of the desired ratio of carbon monoxide to carbon dioxide, and since this ratio is critical, depending upon the carbon content and temperature of treatment, the desired results in annealing are more readily attained by the practice of the invention as herein described.

Referring to the drawing, indicates a source of supply of nitrogen and oxygen. This preferably is a liquefaction column in which air is liquefied and rectified to afford a supply of the desired gases. The efiiuent nitrogen from such a column may and usually does contain varying proportions of oxygen. A gas containing upward to of oxygen may be employed readily where, as in the earlier method hereinbefore mentioned, it is necessary to restrict the oxygen content to not over about 5%. In the present procedure, the proportion of oxygen in the nitrogen is not critical because the composition of the resulting combustion gases is corrected to give the proper ratio of carbon monoxide and carbon dioxide for the particular operation involved.

The mixture of nitrogen and oxygen is delivered by a pipe 6 to a reactor 7 which is filled with carbon, preferably in the form of charcoal. The reactor is heated in any suitable manner, for example electrically by the use of resistance or by the application of waste hot gas from the annealing furnace. The temperature of the reactor is likewise not critical, but is preferably maintained between 500 and 1200 C. Preferably a flow meter 8 is disposed in the pipe ii in order to regulate the amount of the gaseous mixture delivered to the reactor. The gaseous products of combustion in the reactor are delivered through a pipe 9. They will consist of nitrogen principally, with varying proportions of carbon monoxide and carbon dioxide, depending upon the temperature maintained in the reactor. These gases flow through the pipe 55 to a pipe IE3 and thence to the annealing furnace II. A valve I2 is disposed in the pipe 2 to regulate the flow.

Since the gases from the reactor 7 will rarely contain the proper ratio of carbon monoxide and carbon dioxide for the treatment of a steel of particular carbon content at a particular an nealing temperature, it is necessary to modify the ratio. This is accomplished by withdrawing portions of the gas and subjecting them to analysis. If there is an excess of carbon dioxide, the gas is withdrawn through a pipe 23 controlled by a valve M and delivered to an absorber l5 which may be filled with any suitable material adapted to absorb carbon dioxide. There are numerous materials suitable for this purpose, including potassium hydroxide, anhydrous sodium oxide, soda lime, Ascarite and monoethanolamine. After passing through the absorber, the gas is delivered through a flow meter l6 and valve ll to a pipe it connected to the pipe It. By thus absorbing carbon dioxide from a portion of the gas from the reactor 1, the ratio of carbon monoxide to carbon dioxide nay be varied at will to afford the proper atmosphere.

Similarly, if the carbon monoxide is in excess,

. a portion of the gas is withdrawn through a pipe Iii controlled by a valve 22 and delivered to an absorber 2|. A suitable material for the absorption of carbon monoxide is cuprous chloride, acid or basic. Other materials may, however, be employed. From the absorber 2!, the gas is delivered through a flow meter 22 and valve 23 and thence to the pipe l3 which permits it to mingle with the gas flowing through the pipe ID to the annealing furnace H.

Thus, by absorbing a portion of the carbon monoxide or carbon dioxide, depending upon which is in excess, the composition of the gas delivered to the annealing furnace may be varied at will and readily adjusted to aiford the desirable ratio for the particular purpose. The operator has merely to determine by analysis in the usual manner which gas is in excess, and then to manipulate valves to direct the desired proportion of the combustion product through one or the other of the absorbers l5 and 2i to secure a gas of the preferred composition.

Various changes may be made in the details of procedure and in the apparatus as described without departing from the invention or sacrificing the advantages thereof.

I claim:

1. In the method of annealing steel of a predetermined carbon content in which the steel is heated at a predetermined temperature while in contact with a gas containing N2, and CO and CO2 in a desired predetermined ratio; the improvement in forming said gas which comprises selectively absorbing from a portion of a gas containing N2, and CO and CO2 in a ratio other than the desired predetermined ratio the carbonoxygen constituent which is present in excess of the desired ratio and mixing the gas depleted in said constituent with the remainder of the gas, the amount of said constituent which is selectively absorbed being such that the gas resulting from said mixing will contain CO and CO2 in the desired predetermined ratio.

2. In the method of annealing a steel of a predetermined carbon content in which the steel is heated at a predetermined temperature while in contact with a gas containing N2, and CO and CO2 in a desired predetermined ratio; the improvement in forming said gas which comprises passing N2 and 02 through a bed of carbon at an elevated temperature between 566 C. and 1200 C. with resultant formation of a gas containing N2, and CO and CO2 in a ratio other than in the desired predetermined ratio, selectively absorbing from a portion of said gas the carbon-oxygen constituent which is present in excess of the desired ratio, and mixing the gas depleted in said constituent with the remainder of the gas, the amount 01' said constituent which is selectively absorbed being such that the gas resulting from said mixing will contain CO and CO2 in the desired predetermined ratio.

HAIM H. CHISVVIK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 109,276 Cummins et a1. Aug. 20, 1889 2,085,597 Marshall June 29, 1937 OTHER REFERENCES Controlled Atmospheres for the Heat Treatment of Metals, pages 205-210, by Jenkins, 1946. Controlled Atmospheres. American. Society for Metals, pages l-l2, 1942. 

1. IN THE METHOD OF ANNEALING STEEL OF A PREDETERMINED CARBON CONTENT IN WHICH THE STELL IS HEATED AT A PREDETERMINED TEMPERATURE WHILE IN CONTACT WITH A GAS CONTAINING N2, AND CO AND CO2 IN A DESIRED PREDETERMINED RATIO; THE IMPROVEMENT IN FORMING SAID GAS WHICH COMPRISES SELECTIVELY ABSORBING FROM A PORTION OF A GAS CONTAINING N2, AND CO AND CO2 IN A RATIO OTHER THAN THE DESIRED PREDETERMINED RATIO THE CARBONOXYGEN CONSTITUENT WHICH IS PRESENT IN EXCESS OF THE DESIRED RATIO AND MIXING THE GAS DEPLETED IN SAID CONSTITUENT WITH THE REMAINDER OF THE GAS, THE AMOUNT OF SAID CONSTITUENT WHICH IS SELECTIVELY ABSORBED BEING SUCH THAT THE GAS RESULTING FROM SAID MIXING WILL CONTAIN CO AND CO2 IN THE DESIRED PREDETERMINED RATIO. 